In rotor aircraft, such as helicopters and prop planes, rotor modulation of received radio signals has been a continuing problem in the VOR/ILS frequency range, commonly 108-118 megahertz. In order to be certified for IFR, the aircraft must have an antenna system which has an omnidirectional radiation pattern and can receive the instrument landing information over the transmitted frequency without being garbled by rotor modulation or coupling. Prop planes have employed a dipole antenna bent in a V-shape to provide an omnidirectional radiation pattern, and have changed the RPM of the rotor in order to eliminate modulation when receiving instrument landing information. Helicopters have a much smaller permissible RPM window (commonly around 4%) and thus cannot change rotor RPM enough to eliminate rotor modulation, and hence have not been certified for IFR.